Method and system for inflight refueling of unmanned aerial vehicles

ABSTRACT

A system and method for refueling unmanned aerial vehicles. The system is adapted to refuel a first unmanned aerial vehicle from a second unmanned aerial vehicle and includes an arrangement for flying the first and second vehicles to proximity within a predetermined range and for connecting an umbilical from the second vehicle to the first vehicle in flight. In the illustrative embodiment, the arrangement for connecting includes a targeting system for electromagnetically detecting a refueling receptacle on the first vehicle. The targeting system includes a first coil around a refueling receptacle on the first vehicle. A seeker is disposed at a first end of said umbilical on the second vehicle. The seeker includes three detector coils adapted to detect a magnetic signal from the first coil around the receptacle on the first vehicle. The coils are mounted such that the detector coils point in different directions. The outputs of the coils are processed to determine the direction and range to the UAV from the tanker UAV.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional Application of U.S. patent application filed Aug.13, 2007 by James Small et al., Ser. No. 11/891,959, entitled METHOD ANDSYSTEM FOR INFLIGHT REFUELING OF UNMANNED AERIAL VEHICLES (AttorneyDocket No. PD06W189) the teachings of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aeronautics. More specifically, thepresent invention relates to systems and methods for refueling vehiclesin flight.

2. Description of the Related Art

The use of unmanned air vehicles (UAVs) for various military andcivilian applications is rapidly expanding. A typical UAV flight hasthree parts to its mission: 1) it must be launched from a support baseand fly to an area of operation; 2) it must loiter in its area ofoperation while performing its intended functions; and 3) it must flyback to its support base and land while carrying sufficient spare fuelto account for unforseen delays such as unfavorable headwinds.

At takeoff, a UAV must carry sufficient fuel for all three phases of itsmission. It is often the case that mission parts 1 and 3 will eachconsume as much fuel as mission part 2, which is the useful portion ofits total flight.

If a UAV can be refueled in-flight in its area of operation, asubstantial increase in utility may be achieved. Hence, there is agrowing need for a system or method for refueling UAVs in flight toallow the UAV to remain on-station for extended periods withoutconsuming time and fuel to return to its support base.

Unfortunately, currently, it is generally not feasible for UAVs to berefueled from conventional manned tanker aircraft. There are two primaryreasons. First, most UAVs are much smaller and fly slower thanconventional manned tankers, which have been designed to refuel largejet-powered aircraft. It is necessary that the tanker have a flightperformance roughly comparable to the UAV in order to perform closeformation flight during refueling operations. Specially constructedtanker aircraft will generally be required to refuel most UAVs.

Second, aircrews of manned tanker aircraft are unwilling to permitunmanned aircraft to fly in close formation for safety reasons. Duringmanned refueling operations, skilled pilots are in control of both thetanker and receiving aircraft. There is considerable danger to the humancrews in both aircraft should any collision occur during the extremelyclose formation flight. Pilots of both aircraft place a very high degreeof trust in the skill and competence of the other pilot. They areunwilling to rely on the response of robotic unmanned vehicles that maynot be able to react to unforeseen problems. An unmanned tanker aircraftwill generally be required for in-flight refueling of UAVs.

Hence, a need remains in the art for a safe and cost-effective system ormethod for refueling a UAV in flight.

SUMMARY OF THE INVENTION

The need in the art is addressed by the system and method for refuelingunmanned aerial vehicles of the present invention. In the systemimplementation the invention is adapted to refuel a first unmannedaerial vehicle from a second unmanned aerial vehicle and includes anarrangement for flying the first and second vehicles to proximity withina predetermined range and for connecting an umbilical from the secondvehicle to the first vehicle in flight using a novel magnetic targetingsystem.

In the illustrative embodiment, the targeting system includes a firstcoil around a refueling receptacle on the first vehicle. A seeker isdisposed at a first end of said umbilical on the second vehicle. Theseeker includes multiple detector coils adapted to detect a magneticsignal from the first coil around the receptacle on the first vehicle.The coils are mounted such that the detector coils point in differentdirections. The outputs of the coils are processed to determine thedirection and range to the mission UAV from the tanker UAV.

In the illustrative embodiment, the inventive method includes the stepsof flying the first and second vehicles to proximity within apredetermined range and connecting an umbilical from the second vehicleto the first vehicle in flight using the magnetic targeting system. Moregenerally, a targeting method is disclosed including steps of: providinga plurality of coils for detecting a magnetic field; pointing each ofsaid coils for optimal sensitivity of said field in a differentdirection; and processing signals output by said coils to locate atarget. In the illustrative embodiment, the targeting system includes acoil disposed around a target on a first platform; an arrangement foractivating the coil; and an arrangement disposed on a second platformfor sensing a field radiated by the coil.

A novel detector arrangement is also disclosed. The novel detectorincludes a plurality of coils for detecting a magnetic field; anarrangement for pointing each of the coils for optimal sensitivity ofthe field in a different direction; and an arrangement for processingsignals output by the coils to determine a location of an object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified aerial view showing two unmanned aerial vehiclesin flight flying in close proximity to effect autonomous refueling inaccordance with an illustrative embodiment of the present teachings.

FIG. 2 is a diagram that illustrates the generation of a magnetic fieldby an energized coil.

FIG. 3 is a top view of a section of the mission UAV of FIG. 1 in afirst embodiment of the coil thereof in accordance with the presentteachings.

FIG. 4 is a side view the mission UAV of FIG. 1 in a second embodimentin accordance with the present teachings by which the receptor coil isdisposed in a basket coupled to the UAV via a flexible fuel line.

FIG. 5 is an end view of an illustrative embodiment of the basket ofFIG. 4.

FIG. 6 is a simplified diagram of the fuel probe seeker of the system ofFIG. 1 in accordance with the present teachings.

FIG. 7 is a sectional side view of a portion of the seeker of FIG. 1 inaccordance with an illustrative embodiment of the present teachings.

FIG. 8 is a simplified block diagram of the electrical subsystem of themission UAV in accordance with an illustrative embodiment of the presentteachings.

FIG. 9 is a simplified diagram of the seeker electronics of anillustrative implementation of the UAV tanker seeker/targeting system ofthe present invention.

FIG. 10 is a block diagram of an illustrative implementation of a UAVtanker seeker precision guidance computer in accordance with anillustrative embodiment of the present teachings.

FIG. 11 is a flow diagram of an illustrative embodiment of a method forrefueling an unmanned aerial vehicle in accordance with the presentteachings.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

FIG. 1 is a simplified aerial view showing two unmanned aerial vehiclesin flight flying in close proximity to effect autonomous refueling inaccordance with an illustrative embodiment of the present teachings. Asshown in FIG. 1, the system 10 includes a mission UAV 12, a tanker UAV14 and a novel magnetic targeting arrangement 16 for guiding anumbilical 18 to effect a refueling coupling between the two vehicles.Each UAV has an airframe, control surfaces and guidance, navigation,communication and propulsion systems as is common in the art.

As per conventional teachings, the mission UAV 12 has the followingsystems:

-   -   1. Mission Package: Mission Data Link, sensors, other payloads        (i.e. a broadcast transmitter, guided missiles, etc).    -   2. Flight Systems: engine and fuel management, flight        instruments, flight control servos, autopilot computer.    -   3. Navigation Systems: GPS receiver, inertial navigator, air        traffic radar transponder, collision avoidance systems.    -   4. Flight Management Computer:        -   Contains pre-programmed course, waypoints, altitude, and            speed information.        -   Directs the autopilot, engine, takeoff and landing sequence,            collision avoidance maneuvers, etc.        -   Monitors fuel consumption, electric power, and other vehicle            health information.        -   Communicates with the Ground Control Operator and receives            mission change instructions.        -   Executes fail-safe maneuvers if control links are lost.    -   5. Control Data Links:        -   May include two or more redundant radio links from the UAV            to the Ground Control pilot-operator.        -   Includes transmitters, receivers, antennas, possibly a            satellite tracking antenna.        -   Also provides for transmission security such as encryption,            passwords, bit error checking.        -   Provides a two-way voice channel to the Ground Controller            for air-to-air communication.    -   6. Air Traffic Radios:        -   Typically the Ground Controller is able to communicate from            the UAV to other aircraft and Air Traffic Control.        -   Conventional aircraft radios are installed on the UAV. They            are operated through the Control Data Link.

For mid-air refueling in accordance with the present teachings, themission UAV 12 will have the following additional systems:

-   -   1. a refueling port:        -   a fixed port on the UAV structure or alternately a flexible            hose and basket which are deployed during refueling;    -   2. software in the flight management computer to communicate        position information to the tanker UAV; and    -   3. a precision guidance system to allow an unmanned tanker to        connect to the fuel port.

In accordance with the present teachings, the tanker UAV 14 operates asanother UAV in the air space and has systems similar to the mission UAV12 including:

-   -   1. a ground control station and human pilot-operator;    -   2. the mission package will include a large fuel tank, pumps,        metering sensors, and a deployable fuel probe; and    -   3. flight systems, navigation systems, flight management        computer, control data links, air traffic radios.

In addition, as discussed more fully below, the tanker is designed tolocate the mission UAV, intercept its course, join up in closeformation, and maneuver its fuel probe to connect with the fuel port onthe mission UAV.

In addition, the tanker includes:

a system for precision maneuvering of a fuel probe either by maneuveringthe entire aircraft and/or separately steering the probe;

a precision guidance system to direct the fuel probe to the mission UAVfuel port; and

probe sensors to detect mechanical strains after a mechanical latch hasbeen achieved.

In accordance with the invention, when the mission UAV 12 is in need offuel, it will enter a standardized holding pattern that has beenapproved by an air traffic controller. As is common in the art, theholding pattern may be a racetrack path flown at constant altitude in ablock of airspace that has been cleared of other aircraft operations.The unmanned tanker aircraft 14 will be directed to the holding area byits ground control station. The receiving mission UAV 12 maycontinuously transmit its position and altitude information by radio.The position information may be derived from an onboard satellitenavigation receiver such as a Global Positioning Satellite (GPS)receiver.

The mission computer on board the tanker UAV 14 compares the receivedinformation to its own position and calculates a safe intercept course.For example, the tanker 14 may approach the receiver from above andslightly ahead as shown in FIG. 1. Many other approach configurationsare possible.

Once the tanker and receiver are in loose formation flight, the tankerwill transition to close formation flying. In general, the tankeraircraft will be equipped with special systems to permit both loose andclose formation flight. By placing most specialized systems in thetanker 14, the receiving UAV 12 will require minimal modification topermit it to participate in in-flight refueling. Since a tanker aircraftmay service multiple receiving aircraft, it is cost effective to placespecialized systems mostly in the tanker.

Loose formation flying may be defined as coordinated flight between twoaircraft which can be accomplished by reference to external radionavigation aides such as the GPS system. Close formation flight may bedefined as the extreme positional accuracy required for the tanker UAVto connect its refueling probe to a receptacle on the receiving UAV. Ingeneral, the tanker must maneuver its refueling probe to close proximity(e.g. within approximately 2 centimeters of the receiving receptacle) inorder to achieve a mechanical latch. The tanker must then maintain closeformation flight during the transfer of fuel. This extreme precisionmust be accomplished while both aircraft are subject to unpredictedchanges in winds and turbulent air currents.

Close formation flying may be achieved by providing a cooperative targeton the receiving UAV and a matched seeker mechanism on the tanker UAV.Many embodiments of cooperative targets are possible. For example, thereceiving target may emit radio signals, optical signals, magneticfields, electric fields, radioactive emissions, or acoustic signals.From these emissions, the seeker on the tanker is able to derivedirection and range information to the cooperative target. Thisinformation is used by a computer on the tanker for two functions. Itprovides guidance information to flight controls on the tanker aircraftto maintain close flight. It also provides guidance information used toindependently maneuver the refueling probe as it approaches thereceiving receptacle

As a practical matter, many of the above listed emissions are unsuitableor have serious limitations for seekers for unmanned aerial refuelingprobes. For example, it would seem reasonable for the target aircraft toradiate a radio wave that the tanker could use as a homing signal. Inpractice, radio waves are not a good choice for very close homingdistances. When radio (or microwave) waves are radiated, they aresubject to strong multipath reflections from various parts of the nearbyaircraft body. Furthermore, as the seeker antenna approaches a radiowave source, it merges the near field patterns and side lobes of boththe transmitting and receiving antennas. The result is confused andrapidly changing apparent directions to the target.

Another reasonable seeker approach might be to use optical sources anddetectors. The target could be provided with flashing lights and theseeker can use well-known optical imaging methods to provide guidancesignals. As a practical matter, the optical seeker suffers from severaldeficiencies. Most simple optical systems have a limited field of view.The complexity of the optical system rises rapidly when it is requiredto search a very large field of view to find the active target. Mostimportantly, optical systems are very easily disrupted by fog, rain,water drops on the optical surfaces, mud and oil that may be common inaircraft operations.

In accordance with the present teachings, a coil 20 is provided around areceptacle 22 of the mission UAV 12. The coil 20 is powered with anelectrical current and emits a magnetic field in response thereto. Thisis illustrated in FIG. 2. The magnetic field is sensed by a seeker 30disposed at the end of the umbilical (fuel probe) 18 of the tanker UAV14. In the preferred embodiment, the seeker 30 includes multipledetector coils (not shown in FIG. 1) that sense the magnetic fieldemanated by the receptor coil 20.

The signals are processed to provide range and direction precisionguidance commands to a mechanism to maneuver the umbilical as discussedmore fully below. In FIG. 1, the mechanism is a boom 32.

FIG. 2 is a diagram that illustrates the generation of a magnetic fieldby an energized coil. FIG. 2 shows a cooperative target and seeker basedon the principle of magnetic induction. When a current flows through awire loop or coil, a dipole magnetic field pattern develops in the nearspace surrounding the coil. If the current flowing through the coil isalternating current, then an alternating dipole magnetic field isgenerated in the vicinity of the coil. The alternating magnetic fieldreadily induces voltages and currents in any nearby unpowered coils,which may be used in a seeker. It is important to note that the dipolemagnetic field is not a radiated radio wave. With dipole sources, themagnetic field strength drops very rapidly with distance from the sourcecoil. In general, the magnetic field strength decreases with distance Rfrom the energized coil by a factor of (I/R)³. That is, magneticinduction signals drop off in signal strength approximately as1/(range³). This is a very rapid drop.

Since the signal strength drops so rapidly with distance, the range tothe target may be estimated quite accurately by simply measuring thestrength of the detected signal. If the source strength of the target isheld constant, measuring the received strength gives a very goodestimate of the range to the target. Unlike radar systems, it is notnecessary to send two-way signals to measure range. With the coildimensions and drive current shown in FIG. 2, the target can be locateda distance of (e.g. 15 meters) from the seeker. In accordance with thepresent teachings, a seeker uses plural detector coils to determinedirection to the target in two dimensions. Each coil points in adifferent direction. The coil which is most closely aligned with thesource produces the strongest signal. When the signal is equallybalanced in all coils, the target lies directly ahead. The seeker logic(discussed below) provides output signals to adjust the flight controlsand to maneuver the refueling probe.

Hence, the coil 20 is activated by an alternating current from agenerator 24. In an illustrative embodiment, the coil 20 has 10 turns,of a suitable conductor such as 18 gauge copper wire, wrapped on adielectric or air core of diameter of approximately 60 centimeters andthe generator 24 outputs an alternating current of approximately 100milli-amperes at 5 kilohertz (kHz). Those skilled in the art willappreciate that the present invention is not limited to the coil designor the level or frequency of the power applied thereto.

FIG. 3 is a top view of a section of the mission UAV 12 of FIG. 1 in afirst embodiment of the coil thereof in accordance with the presentteachings. In this embodiment, the coil 20 is disposed around arefueling port or receptacle 22 in an aperture provided in a top surface26 of the airframe of the mission UAV 12. As shown in FIG. 3, in thebest mode, an optional second coil 28 is included for more precisetargeting as discussed more fully below.

FIG. 4 is a side view the mission UAV 12′ of FIG. 1 in a secondembodiment in accordance with the present teachings by which thereceptor coil 20′ is disposed in a basket 32′ coupled to the UAV via aflexible (e.g. rubber) fuel line 34′.

FIG. 5 is an end view of an illustrative embodiment of the basket 32′ ofFIG. 4. As shown in FIG. 5, the basket 32′ includes a cone 36′fabricated of metal, plastic, or fabric with a fuel port 22′ disposed ata center portion thereof in communication with a fuel reservoir (notshown) on the UAV 12. The first coil 20′ is disposed at a distal end ofthe basket 32′ relative to the fuel port 22′. A second coil 28′ isdisposed between the first coil 20′ and the fuel port 22′. Those skilledin the art will appreciate that the invention is not limited to thefabrication of the basket. That is, the basket 32′ may be of a solid,mesh or web construction and/or shaped to fly in a desired mannerwithout departing from the scope of the present teachings.

FIG. 6 is a simplified diagram of the fuel probe seeker of the system ofFIG. 1 in accordance with the present teachings. In the best mode, theseeker 30 includes four detector coils 40-43 (of which only two areshown in FIG. 6). In the illustrative embodiment, each detector coil isimplemented with 400 turns of a suitable conductor such as 22 gaugecopper wire around an iron core approximately 2 centimeters in diameter.The detector coils are mounted to point in separate directions. Thedetector coils sense the magnetic field radiating from the receptorcoils. The output of each detector coil is amplified by an associatedamplifier 44-47 and input to a processor 50. The coil which points mostdirectly toward the receptor coil 20 of the mission UAV will have thelargest signal amplitude. The processor 50 compares the outputs of thedetector coils and calculates the range and direction to the receptorcoil 20 as discussed more fully below. The range and direction processor50 may be implemented in discrete logic circuits, field-programmablegate arrays, application-specific integrated circuits, or other suitablemeans. In the best mode, the processor 50 is implemented in softwareadapted to run on a general-purpose computer (not shown) as discussedmore fully below.

FIG. 7 is a sectional side view of a portion of the seeker of FIG. 1 inaccordance with an illustrative embodiment of the present teachings. Asshown in FIG. 7, the seeker 30 includes four magnetic detector coils40-43 (of which only two 40, 42 are shown in FIG. 7). The coils 40, 42are disposed within a nonmetallic housing 48 at a distal end of the fuelline 18. The amplifiers 44-47 are also disposed within the housing 48.Wiring from the amplifiers is enclosed within a sheath 52 around thefuel line 18 and as discussed more fully below, outputs signals at 5 and7 kHz in an illustrative embodiment thereof.

As is common in the art, the fuel line 18 has a ball and spring checkvalve arrangement 60 at the distal end thereof. The spring loaded ballcheck valve 60 is designed to open the line 18 and permit fuel to flowtherethrough after a successfully latching operation.

FIG. 8 is a simplified block diagram of the electrical subsystem of themission UAV in accordance with an illustrative embodiment of the presentteachings. As shown in FIG. 8, in the preferred embodiment, the missionUAV electrical subsystem 70 includes first and second signal sources 74and 76 which operate under control of a conventional onboard flightmanagement computer 72. The first source 74 drives the first coil 20 andthe second source drives the second (inner) coil 76. In the illustrativeembodiment, the first and second sources 74 and 76 are oscillatorsoperating at 5 kHz and 7 kHz, respectively.

FIG. 9 is a simplified diagram of the seeker electronics of anillustrative implementation of the UAV tanker seeker/targeting system ofthe present invention. As shown in FIG. 9, the seeker electronicsubsystem 80 includes a set of bandpass filters (e.g. 82, 84) for eachdetector coil (of which only two 40, 42 are shown in FIG. 9). Eachbandpass filter (BPF) is coupled to an associated power detector 90, 92,94 or 96. It should be noted that this embodiment has four detectorcoils and four amplifiers are included along with eight bandpass filtersand eight power detectors. Nonetheless, the invention is not limited tothe number of detectors or the arrangement by which the detector outputsare processed. Each power detector outputs a voltage proportional to thepower level of the signal detected by the associated coil.

In FIG. 9, the first and third detectors illustrated 40 and 42 arecoaxial for the purpose of discussion. It should be understood that thesecond and fourth detectors 41 and 43 (not shown) are also coaxial withrespect to an axis that is orthogonal with respect to that of the firstand third detectors and coupled an identical circuit 80. Hence, thefollowing discussion with respect to signal processing will beunderstood to apply to both sets of detectors. The magnetic signal fromeach receptor coil 20, 28 as detected by each detector coil 40 is summedwith the signal from the other detector coil 42 for a given axis todetermine range by first and second summing amplifiers 91 and 97.Likewise, the magnetic signal from each receptor coil 20, 28 as detectedby each detector coil 40 is subtracted from the signal from the otherdetector coil 42 for a given axis to determine direction by first andsecond summing amplifiers 91 and 97. The range and direction values fromthe sum and difference amplifiers for each axis, provided by each pairof detector coils 40/42 and 41/43, are fed into a precision guidancecomputer 100 illustrated in FIG. 10.

FIG. 10 is a block diagram of an illustrative implementation of a UAVtanker seeker precision guidance computer in accordance with anillustrative embodiment of the present teachings. The computer 100includes an analog-to-digital A/D converter 102, 104, 106, and 108 foreach of the amplifiers 91, 93, 95 and 97, respectively, of FIG. 9. Next,a first processor 110 estimates range with respect to the magneticsignal detected from the outer coil 20 of FIG. 8 and a second processor116 estimates range with respect to the magnetic signal detected fromthe inner coil 28 of FIG. 8. Likewise, third and fourth processors 112and 114 ascertain angle with respect to the magnetic signal detectedfrom the inner coil 28.

Next, the range and angle outputs with respect to the outer coil 20 areprocessed (118) to ascertain speed and steering commands for the closingrate of the seeker 30 from the autopilot computer 132. Likewise, therange and angle outputs with respect to the inner coil 28 are processed(120) to ascertain speed and steering commands for the short-rangeclosing rate of the seeker 30 from the autopilot computer 132.

When a predetermined short range to target is detected, a signal (122)is output which activates a switch 124, which, in turn, selects theshort-range speed and maneuvering commands (122) for input to theautopilot computer 126. Autopilot operation is enabled by a signal (128)from a conventional onboard flight management computer 130 on detectionof sufficient proximity to activate the autopilot 126. The flightmanagement computer 130 is coupled to an onboard communication system132. The autopilot 126 provides short-range guidance commands for themaneuvering fins 54 and 56 of the seeker 30 (FIG. 7) to effect asuccessful docking of the seeker probe 30 from the tanker UAV 14 withthe refueling port receptacle 22 of the mission UAV 12 (FIG. 1).

FIG. 11 is a flow diagram of an illustrative embodiment of a method forrefueling an unmanned aerial vehicle in accordance with the presentteachings. At steps 202 and 204, a refueling command is issued to themission UAV 12 and the tanker UAV 14 by timed pre-program or commandfrom ground control. At step 206, the mission UAV flies to and joins acontinuous racetrack course around designated GPS waypoints. At step208, the tanker UAV flies to and joins the continuous racetrack coursearound the designated GPS waypoints at a safely higher altitude. At step210, the mission UAV transmits an identification code and local GPSposition coordinates by air-to-air radio at predetermined (e.g. 30second) intervals and listens for a tanker reply. At step 212, thetanker UAV listens for mission UAV position reports. On receipt of aposition report, the tanker UAV sends a reply. On receipt of the reply(step 216), the mission UAV increases the GPS position report intervalto 3 seconds, for example, and at step 224 activates the fuel portprecision guidance magnetic beacon (coils 20 and 28). At step 218, thetanker detects the increased frequency of position reports and at step220 uses the mission UAV position reports and the expected racetrackcourse to compute an intercept path to arrive at safe (e.g. 10 meter)distance above and behind the target.

Then, at step 222, the tanker deploys the refueling probe 16 (FIG. 1)and activates precision guidance detectors 40-43 (FIG. 9). At step 226,the tanker checks for predetermined (e.g. 15 meter) proximity. If theproximity threshold is detected, then at step 230, the tanker switchesfrom GPS guidance to precision guidance using signals from the magneticfuel probe seeker (30) and at step 232 closes in range until mechanicallock (successful docking) is achieved. The mechanical lock is detectedby the mission UAV (step 234) and the two UAVs hold close formationusing fuel probe strain gauge signals. At step 236, the mission UAVstops radio transmission of GPS position reports and, at step 238, sendsa signal to start the flow of fuel. On receipt of the ‘start fuel’signal, the tanker transfer fuel pumps are activated until receipt of a‘stop’ signal from the mission UAV or the mission UAV disconnects orwhen an expected load is reached or some other preprogrammed stopcondition (step 242). When its tanks are full, the mission UAV sends asignal to stop the flow of fuel (step 240) and disconnects the refuelingprobe (step 244). At steps 246, 248 and 250, on detection of amechanical disconnect, the tanker maneuvers safely away from the missionUAV and resumes normal navigation. At step 252, the tanker sends a‘tanker clear’ signal to the mission UAV. At steps 254, 256 and 258 themission UAV waits for the ‘tanker clear’ message, deactivates the fuelport precision guidance beacon and resumes normal navigation.

In the method of FIG. 11, each step is reported by a data link to a UAVground control station. The calculations are performed by each UAV'sassociated flight management computer.

Thus, the present invention has been described herein with reference toa particular embodiment for a particular application. Those havingordinary skill in the art and access to the present teachings willrecognize additional modifications applications and embodiments withinthe scope thereof.

It is therefore intended by the appended claims to cover any and allsuch applications, modifications and embodiments within the scope of thepresent invention.

Accordingly,

1. An unmanned aerial vehicle comprising: an airframe; guidance meansdisposed on said airframe; and a seeker disposed on said airframe forfinding a target by sensing a magnetic field emanating therefrom.
 2. Anunmanned vehicle comprising: an airframe; guidance means disposed onsaid airframe; and a seeker coupled to said guidance means for finding amagnetic target.
 3. The invention of claim 2 wherein said target is acoil.
 4. The invention of claim 3 wherein said coil is disposed around areceptacle.
 5. The invention of claim 4 wherein said receptacle isdisposed on a second vehicle.
 6. The invention of claim 5 wherein saidsecond vehicle is unmanned.
 7. The invention of claim 2 furtherincluding means disposed on said unmanned vehicle for retaining a fluid.8. The invention of claim 7 wherein said fluid is fuel.
 9. The inventionof claim 2 further including a communication system disposed on saidairframe.
 10. The invention of claim 2 further including a propulsionsystem disposed on said airframe.
 11. The invention of claim 2 whereinsaid guidance means further includes a seeker autopilot.
 12. An unmannedaerial vehicle comprising: an airframe; control surfaces disposed onsaid airframe; a guidance system disposed on said airframe andoperationally coupled to said control surfaces; a propulsion systemdisposed on said airframe; a receptacle disposed on said airframe; andan coil of conductive material disposed around said airframe.
 13. Theinvention of claim 12 further including means for supplying anelectrical signal to said coil.
 14. The invention of claim 12 furtherincluding a communication system disposed on said airframe.
 15. Atargeting system comprising: a coil disposed around a target on a firstplatform; means for activating said coil; and means disposed on a secondplatform for sensing a field radiated by said coil.
 16. The invention ofclaim 15 wherein said means for sensing includes a detector arrangement.17. The invention of claim 16 wherein said means for sensing includesmeans for processing a signal output by said detector arrangement todetermine a range and a direction of said coil.
 18. A detectorarrangement comprising: a plurality of coils for detecting a magneticfield; means for pointing each of said coils for optimal sensitivity ofsaid field in a different direction; and means for processing signalsoutput by said coils.
 19. A targeting method including the steps of:providing a plurality of coils for detecting a magnetic field; pointingeach of said coils for optimal sensitivity of said field in a differentdirection; and processing signals output by said coils to locate atarget.
 20. A method for refueling a first unmanned aerial vehicle froma second unmanned aerial vehicle including the steps of: flying saidfirst and second vehicles to proximity within a predetermined range andconnecting an umbilical from said second vehicle to said first vehiclein flight using a magnetic targeting system.